Going by your previous thread, the niche you're looking towards is highly saturated and has tough competition.
To make a board that lives by itself you need quality members and a reason for members to stay and contribute back. Original Content
You can go for News about upcoming projects big companies have, Google I/O news, something that's trending on Kickstarter, or even DIYs
But just as AOKI said, do somehting you are experienced with
The niche is important to get people there.. the community is important to get people coming back. The content is what will get your site found.
If you are trying to start a forum about a saturated niche, then narrow it down.. for example instead of doing a hacking forum.. focus it on a specific niche within hacking.. whether its program based, web based, certain tools, OS's, methods, etc.. You will also want to find people knowledgeable in said niche or category and pitch them the forum, get them to be on board.. noone is going to join a forum with low members and no content.. Have the "smarter" people start posting and contributing.. work on the SEO... the word of mouth... etc and slowly you will start getting members in.. and if your site is good, you will make a great community